The SYREN was invented by Cagniard de Latour. It consists of a metal cylinder, a tube passes through the bottom, and through the tube air is blown into the cylinder. On the top a number of holes are drilled, while just over the cylinder top, almost in contact with it, is a metallic disc, which rotates upon a vertical axis. The disc is perforated with holes equal in number to those in the cylinder top, but the holes are not perpendicular, they slope in opposite directions. So when the air is forced through the holes in the top of the cylinder it impinges upon one side of the holes in the rotating disc, and blows it round. The disc in one revolution will therefore open and shut as many holes as there are in the disc and cylinder, and the air blown in will escape in so many puffs—the number of puffs in a given time depending upon the rapidity of rotation. There is an arrangement to show the number of turns. By these rotations a sound is produced which rises in pitch as the revolutions are increased in number. [Illustration: Fig. 198. Sound Figures. Fig. 199.] To determine the pitch of a certain sound we must find the number of times the plate revolves in that time, then we shall have the number of vibrations per second required to produce the note we desire. The arrangement working in a notched wheel tells us the number of rotations of the disc. Successive, and rapidly-successive puffs or beats are heard as the rotation increases, and at length the two sounds will disappear, and merge into one, which is perhaps that of the tuning-fork, whose note you require to find the “pitch” of. By maintaining this rate for a minute or less, and setting the gear to tell the revolutions, the number will be found marked on the dial of the apparatus. So by multiplying the number of revolutions of the disc by the number of the holes, and dividing the product by the number of seconds during which the disc was in connection with the recording gear, we shall have the number of vibrations per second necessary to produce the pitch corresponding to the given sound. The above is the description of one form of Syren; there are others, which, however, we need not detail. We have seen that there are certain _nodal points_, or resting-places, in vibrations, and this can easily be shown upon a fiddle-string, from which paper discs will fall off except on the nodal point, showing that there is no vibration there. The same experiment may be made by means of plates, which will give us what are termed Chladni’s figures. Suppose we strew a glass-plate with fine sand, and stroke the edge with a fiddle-bow. The vibrations of the plates will make certain patterns, and cast the sand upon those points of repose to form nodal lines in various directions. The plates must, of course, be held or fastened, and a variety of _sound figures_ may be produced. (_See_ figs. 198 and 199.)